1. Field of the Invention
The present invention relates to a polishing apparatus, and more particularly to a polishing apparatus for polishing and planarizing a surface of a substrate, such as a wafer, while preventing defects that could be caused by particles contained in a polishing liquid or other substances attached to processing mechanisms disposed in the polishing apparatus.
2. Description of the Related Art
A polishing apparatus for polishing a surface of a wafer typically has therein various types of processing mechanisms including a polishing table having a polishing surface formed by a polishing pad and a polishing head (top ring) for holding the wafer. The wafer is held by the polishing head and pressed at a predetermined pressure against the polishing surface of the polishing pad, while the polishing table and the polishing head are moved relative to each other. As a result, the wafer is placed in sliding contact with the polishing surface, so that the surface of the wafer is polished to a flat mirror finish. In chemical mechanical polishing (CMP), a polishing liquid (i.e., slurry) containing fine particles therein is supplied onto the polishing surface during polishing of the wafer. After polishing, the wafer is transported by a transporter to a cleaning unit and a drying unit, where the polished wafer is cleaned and then dried. Thereafter, the wafer is removed from the polishing apparatus.
When the substrate, such as wafer, is polished while the polishing liquid is supplied, a large amount of polishing liquid and particles (e.g., polishing debris) remain on the polishing surface of the polishing table. Moreover, during polishing, the polishing liquid is scattered around the polishing table and may be attached to the processing mechanisms arranged around the polishing table. Further, the polishing liquid may be attached to a transporting unit for transporting the polished substrate and a polishing tool of the cleaning unit for cleaning the surface of the polished substrate. If the polishing liquid and the polishing debris remain on the polishing surface of the polishing table and/or if the polishing liquid is attached to the processing mechanisms around the polishing table and the cleaning tool of the cleaning unit, defects of the polished substrate may occur.
Typically, various types of cleaning units are provided at predetermined locations in the polishing apparatus. These cleaning units have jet orifices that eject a cleaning liquid periodically toward predetermined portions of the polishing apparatus so as to wash away the polishing liquid attached to the polishing table and the mechanisms around the table. Such a cleaning liquid may typically be deaerated pure water supplied from a factory into the polishing apparatus.
An ultrasonic cleaning unit is known as the cleaning unit provided in the apparatus. This ultrasonic cleaning unit uses high-pressure water with cavitation for cleaning the polishing apparatus. The deaerated pure water (i.e., cleaning liquid) supplied from the factory into the polishing apparatus is typically used as the high-pressure water of the ultrasonic cleaning unit.
The deaerated pure water (i.e., cleaning liquid) supplied from the factory into the polishing apparatus contains very little gas therein. For example, a concentration of dissolved oxygen in the deaerated pure water (i.e., DO value) is typically at most 20 ppb, and may be even controlled to at most 5 ppb. Fabrication of state-of-the-art devices may require use of the pure water having a dissolved-oxygen concentration of 1 ppb.
The ultrasonic cleaning process utilizing the cavitation is a physical cleaning process that uses a gas-containing liquid that has been processed by ultrasonic wave. An example of a specific condition of the dissolved gas required for the liquid that is to be supplied to the ultrasonic cleaning unit is that “the concentration of the dissolved gas in the liquid is in a range of 1 ppm to 15 ppm”. It is also known that, if an excessive amount of gas is dissolved in the liquid for use in the ultrasonic cleaning process, sufficient cleaning properties cannot be obtained.
As described above, when the deaerated pure water with the DO value of at most 20 ppb is used in the ultrasonic cleaning process, it is difficult to obtain sufficient cleaning properties because the pure water contains very little dissolved gas. Accordingly, in the cleaning process for the apparatus that is conducted under particle contamination due to the polishing liquid, the use of the deaerated pure water may prevent the ultrasonic cleaning process from achieving full advantages of its cleaning effect.